For example, in a power device, in order to drive a three-phase AC motor or the like, an inverter converting a voltage from DC to AC is employed. In the inverter, a high voltage applied to an AC motor should electrically be isolated from a control unit, and a photocoupler has conventionally been employed as an isolation element.
As a transformer is reduced in size and thickness in recent years, however, a photocoupler is replaced with a pulse transformer superior in reliability, power consumption, integration, and transfer speed or an isolation element including a capacitance. A pulse transformer is required not only to achieve isolation but also to achieve lower cost based on reduction in circuit area.
In addition, for example, since a signal transmission circuit transmitting a signal through an isolation element is included in a three-phase AC motor or the like, incorrect output due to noise from the motor or the like should be suppressed.
Japanese Patent Laying-Open No. 7-213057 (PTD 1) discloses an isolation-type element for signal transmission. The isolation-type element for signal transmission disclosed in Japanese Patent Laying-Open No. 7-213057 (PTD 1) is constituted of a thin-film transformer, a first pulse conversion means, and a second pulse conversion means, the second pulse conversion means connected to a secondary winding has an input side connected to a cathode electrode of each of a first diode and a second diode having anode electrodes commonly connected to each other, and a resistor is connected in parallel to each of the first diode and the second diode. Then, an anode electrode common connection point of the first diode and the second diode is connected to respective source electrodes of a first MOS transistor and a second MOS transistor and a ground terminal GND.
In the isolation-type element for signal transmission disclosed in Japanese Patent Laying-Open No. 7-213057 (PTD 1), when a pulse signal which appears in the second winding of the thin-film transformer exhibits positive polarity (an opposite side of the secondary winding exhibits negative polarity), the first diode is biased in a reverse direction and the second diode is biased in a forward direction. Therefore, the second diode is set to an ON state, a voltage across a gate electrode of the first MOS transistor and ground terminal GND is set approximately to 0 V, the first MOS transistor is set to an OFF state, a pulse voltage of positive polarity of the secondary winding is applied substantially across the gate electrode of the second MOS transistor and ground terminal GND, the second MOS transistor is turned on, and a voltage at an output terminal OUT of the second pulse conversion means is set to 0 V.
On the other hand, when a pulse signal which appears in the secondary winding of the thin-film transformer exhibits negative polarity (an opposite side of the secondary winding exhibits positive polarity), the first diode is biased in the forward direction and the second diode is biased in the reverse direction. Therefore, the first diode is set to the ON state, the second diode is set to the OFF state, a voltage across the gate electrode of the second MOS transistor and ground terminal GND is set approximately to 0 V, the second MOS transistor is set to the OFF state, a pulse voltage of the secondary winding is applied substantially across a gate electrode of the first MOS transistor and ground terminal GND, the first MOS transistor is turned on, and a voltage at output terminal OUT of the second pulse conversion means is set to a high voltage.
As above, in the isolation-type element for signal transmission disclosed in Japanese Patent Laying-Open No. 7-213057 (PTD 1), since the secondary winding of the thin-film transformer can be formed from a single winding, decrease in the number of windings can be achieved and a smaller shape of a pulse transformer can be realized.